Prof. Wheatstone on the Physiology of Vision. 505 



will indeed coincide when the sliding pannels are in a variety of 

 different positions, and consequently when viewed under differ- 

 ent inclinations of the optic axes ; but there is only one position 

 in which the binocular image will be immediately seen single, of 

 its proper magnitude, and without fatigue to the eyes, because 

 in this position only the ordinary relations between the magni- 

 tude of the pictures on the retina, the inclination of the optic 

 axes, and the adaptation of the eye to distinct vision at different 

 distances, are preserved. The alteration in the apparent magni- 

 tude of the binocular images, when these usual relations are 

 disturbed, will be discussed in another paper of this series, with 

 a variety of remarkable phenomena depending thereon." 



In 1833, five years before the publication of the memoir just 

 mentioned, these yet unpublished investigations were announced 

 in the third edition of Herbert Mayo's " Outlines of Human 

 Physiology " in the following words : — " Mr. Wheatstone has 

 shown, in a paper he is about to publish, that if by artificial 

 means the usual relations which subsist between the degree of 

 inclination of the optic axes and the visual angle which the object 

 subtends on the retina be disturbed, some extraordinary illusions 

 may be produced. Thus, the magnitude of the image remaining 

 constant on the retina, its apparent size may be made to vary 

 with every alteration of the angular inclination of the optic axes." 



I shall resume, the consideration of the phsenomena of bin- 

 ocular vision with this subject, because the facts I have ascer- 

 tained regarding it are necessary to be understood before enter- 

 ing on the new experiments relating to stereoscopic appearances 

 which I intend to bring forward on the present occasion. 



Under the ordinary conditions of vision, when an object is 

 placed at a certain distance before the eyes, several concurring 

 circumstances remain constant, and they always vary in the same 

 order when the distance of the object is changed. Thus, as we 

 approach the object, or as it is brought nearer to us, the magni- 

 tude of the picture on the retina increases ; the inclination of the 

 optic axes, required to cause the pictures to fall on corresponding 

 places of the retinae, becomes greater ; the divergence of the rays 

 of light proceeding from each point of the object, and which 

 determines the adaptation of the eyes to distinct vision of that 

 point, increases ; and the dissimilarity of the two pictures pro- 

 jected on the retina: also becomes greater. It is important to 

 ascertain in what manner our perception of the magnitude and 

 distance of objects depends on these various circumstances, and 

 to inquire which are the most, and which the least influential in 

 the judgements we form. To advance this inquiry beyond the 

 point to which it has hitherto been brought, it is not sufficient 

 to content ourselves with drawing conclusions from observations 



